Generally, the amplifier circuits are classified into A-class, B-class, AB-class and the like. Most of currently used amplifiers employ these types. Then, the A-class, B-class, AB-class amplifiers have a high linearity, but when realized as a high output amplifier, they need a heat radiation plate owing to occurrence of a vast power loss, so that their volume is increased and efficiency is lowered.
Due to these limitations, there appears a tendency which amplifiers performing the amplification not by a linear operation but by a switching operation are actively researched and developed.
D class amplifier is anticipated to show an efficiency of 90% or so, and accordingly to be advantageous in reducing the product size by remarkably reducing the size of the heat radiation plate due to the decrease of the heat emission.
FIG. 1 is a circuit diagram of a basic D class amplifier.
Referring to FIG. 1, a D class amplifier includes a harmonic square wave oscillator 11 for obtaining a constant period of signal, an integrator 12 for converting to a triangular wave a square wave that is an output signal of the harmonic square wave oscillator 11, a differential amplifier 13 for amplifying an audio input signal and a feedback signal, an adding amplifier 14 for amplifying a difference between an output signal of the differential amplifier 13 and an output signal of the integrator 12, a Schmitt trigger driving part 15 for complementarily operating switching transistors Q1, Q2, a filter for filtering output signals of the switching transistors Q1, Q2, and a speaker 17.
The D class amplifier 10 constructed as above generates a PWM signal by inputting an output signal which is negatively fed back to the differential amplifier 13, and performs the signal amplification by switching the transistors with the generated PWM signal. By doing so, the D class amplifier 10 obtains a high linearity.
Then, the conventional D class amplifier can be used in the analog input D class amplifier by negatively feeding back an analog output signal of the output filter 16, but it cannot be applied to an amplifier which accepts a digital input (PCM signal), which acts as a factor lowering the performance of the conventional D class amplifier. Accordingly, there is currently strongly requested a research and development for improving the D class amplifier which receives the digital input signal.